1. Field of the Invention
The present invention relates to a fastener driving tool, such as a pneumatic nailing machine, for driving a fastener such as a nail, staple, or the like into a workpiece.
2. Description of the Prior Art
Various fastener driving tools are known in the art. Generally, the known fastener driving tools can be divided into two groups. In fastener driving tools of one group, a succession of fasteners are automatically supplied from a magazine and each fastener is driven home into a workpiece by a single power stroke of a fastener driver element. According to the other group of fastener driving tools, one fastener held against a fastener driver element is driven by successive power strokes of a piston connected to the fastener driver element.
FIG. 8 of the accompanying drawings shows a fastener driving tool 1 of the former multifire type. A chain of fasteners 9 is loaded in a magazine 7. When both a switch (A) 2 and a switch 3 are operated, a piston 6 connected to a fastener driver element and slidably fitted in a cylinder 5 is lowered to drive a fastener 9 which has been supplied from the magazine 7 into a position beneath the tip end of the fastener driver element. As the piston 6 is lowered, air in a chamber below the piston 6 is forced to flow into an air chamber 19 through a hole 20 defined in a side wall of the cylinder 5. The cylinder 5 has a smaller hole 21 defined in its side wall and positioned such that the piston 6 is displaced past the smaller hole 21 when the piston 6 is positioned near its lower limit in the cylinder 5. When the piston 6 is moved past the smaller hole 21, compressed air is supplied from a chamber above the piston 6 into the air chamber 19. If either the switch 2 or the switch 3 is released, then compressed air above the piston 6 is discharged from a discharge valve 4, and the piston 6 is elevated back to its upper limit in the cylinder 6 by compressed air stored in the air chamber 19. The fasteners 9 are supplied, one at a time, from the magazine 7 into a drive track 23 below the fastener driver element in response to the operation of the switch 2 and the switch 3.
The conventional fastener driving tool 1 shown in FIG. 8 has the following disadvantages:
(1) While fastener driving tools are generally portable and should be lightweight and compact, the prior fastener driving tool 1 of FIG. 8 is relatively large in size and requires a large amount of energy for driving home a fastener in a single power stroke of the fastener driving element.
(2) The fastener driving tool, which should be constructed to meet safety requirements, may be dangerous if triggered in error because the amount of energy discharged in a single power stroke of the fastener driving element to drive a fastener is substantial.
(3) If a workpiece into which a fastener is to be driven is too hard for the driving energy to drive the fastener fully into the workpiece, then the fastener being driven is stopped halfway as shown in FIG. 9, and the fastener driving tool 1 is repelled upwards under reactive forces. The repelling action of the fastener driving tool will be described in greater detail with reference to FIG. 9. When the switch 2 and the switch 3 are operated, compressed air flows into the chamber above the piston 6 to lower the piston 6 quickly to drive the fastener 9. If the workpiece is hard enough to resist and stop the fastener 9 halfway in the power stroke of the piston 6, the piston 6 stops its downward motion when the fastener 9 is stopped. However, since compressed air is still supplied to the chamber above the piston 6 and the power stroke of the piston 6 is not yet completed, the fastener driving tool 1 itself is repelled upwards under reactive forces from the compressed air in the chamber above the piston 6.
FIG. 10 shows a fastener driving tool of the second category in which a fastener is driven home by successive power strokes of a fastener driver element under the energy of compressed air. In operation, a fastener 9 is placed in a guide 14 in the nosepiece of the fastener driving tool 1. The fastener driving tool is held by hand and the nosepiece is held against a workpiece, whereupon a piston 6 is vertically moved reciprocally to drive the fastener in successive power strokes. Each power stroke is small, e.g., about 3 mm, and hence any repelling motion of the fastener driving tool is small. The guide 14 is vertically movably mounted in the nosepiece so that the fastener 9 will not be dislodged out of the guide 14 even when the fastener driving tool 1 is repelled upwards away from the workpiece.
The fastener driving tool of this type is free of some of the problems of the fastener driving tool shown in FIGS. 8 and 9, but still suffers the following shortcomings:
(1) Since the successive power strokes of the fastener driving tool produce continued vibration during its operation, the hand of the operator will be fatigued if the fastener driving tool is continuously used over a long period of time.
(2) The efficiency is low because a fastener has to be manually placed in the guide 14 each time it is to be driven.
(3) When a fastener is manually placed in the guide 14, inasmuch as the operator's fingers are positioned closely to the fastener driver element, they may get injured if the fastener driving tool is erroneously triggered into operation.